Flat-panel direct digital radiographic (DR) systems and storage phosphor-based computed radiographic (CR) systems provide solid-state imaging systems that are advantaged for many types of X-ray diagnostic imaging. The digital image data that can be directly obtained from these systems can be transmitted, manipulated, displayed or printed, and stored as digital data.
In DR and CR systems, radiation is directed through the subject and impinges upon a detector which is used to form the digital image data, based on the intensity of radiation received at each of a number of pixel locations on the detector. The radiation is modulated by tissue structures of the patient, so that the image data obtained from the detector provides an image of internal tissue structures similar to that obtained from conventional film-based x-ray media.
In different types of x-ray imaging apparatus, the radiation source and radiation detector can be positioned at different angles, suited to the requirements of the type of image that is being obtained. Various angular relationships between source and detector have been found to be particularly advantageous for imaging specific portions of the body and can offer the added benefit of reducing the amount of radiation to which a patient must be exposed.
For X-ray imaging, a collimator, positioned near the X-ray source, provides an aperture of variable size for narrowing the radiation beam, thereby reducing the size of the radiation field to the area being imaged. In a particular embodiment, the collimator has movable horizontal and vertical lead blades, disposed on the sides of the X-ray source, and forms an opening that corresponds to the size of the desired anatomical area and the X-ray sensor.
Generally in digital imaging systems, the larger the image obtained, the greater the amount of image processing required. Factors that impact image processing throughput can include overall image dimensions and pixel spatial and dynamic range resolution. Thus, it is advantageous to electronically identify the region of interest within the image, reducing the size of the imageable area to include substantially only that portion of the anatomy that is of interest (i.e., a region of interest; ROI). A defined region of interest of the image can be more quickly processed and reduces the likelihood of interference from background glare that can be distracting and degrade image appearance when displayed.
In some applications, particularly where the optical center-line of the radiation source is substantially perpendicular to the planar surface of the detector, image cropping can be easily accomplished. This is because values such as collimator opening dimensions, Source-to-Image Distance (SID), and collimator position in the radiation path can be readily determined, allowing for straightforward computation.
However, there are various imaging applications where asymmetrical imaging is required, due to oblique incidence angles of radiation from the radiation source, tilted with respect to the detector surface. For this type of imaging, the issue of ROI definition becomes more complex.
One approach requires the use of sensors, as proposed in U.S. Pat. No. 7,003,145 (Polkus) entitled “Image Cropping for Asymmetrical Imaging”. The rotational orientation angles of the radiation source and its collimator is detected using some type of position sensors. A calculation process then employs these angular values in order to compute appropriate image cropping coordinates.
While approaches such as described in Polkus may provide a solution for image cropping with solid-state radiation detectors, there are drawbacks. For example, this type of approach requires the integration of multiple position-sensing components as part of the overall imaging system. The need for tilt sensors and numerous sensors for reporting the position or orientation of imaging components adds complexity and cost to the equipment, adds concerns regarding calibration, is subject to noise and error, and can potentially compromise the robustness of imaging system design. With a design such as that described in Polkus, improper operation of a single sensing component can make it unlikely to automatically identify the area of interest.
Thus, there is a need for an improved, automated image definition method for use with a flat-panel radiographic apparatus, where the region of interest can be readily determined, particularly without requiring the expense and complexity of angular position sensors.